Permanent magnet material is a very important basic material for the currently hi-tech industry. Due to its high magnetic energy product and coercivity, the third generation of rare earth permanent magnet, known as “the king of magnets”, which is neodymium iron boron (Nd—Fe—B), is widely applied to various fields like computers, automobiles, wind turbines, MRI machines, mobile phones, frequency-converted appliances, audio equipments, etc.
Rare earth refers to the lanthanides in the Periodic Table of Chemical Elements, that is, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and two elements closely related to the 15 lanthanides, that is, Sc, and Y. These 17 elements are collectively named as Rare Earth, or simply, RE or R.
Nd—Fe—B permanent magnet materials consist of sintering and adhering families. The process below is usually adopted when producing a high-performance sintered Nd—Fe—B magnet material:
calculating of ingredient→weighing and preparing of raw materials→vacuum fusing→quick condensing and casting→hydrogen decrepitating and dehydrogenating→airflow pulverizing→mixing→magnetic field orienting and shaping→vacuum sintering and tempering.
Specifically, the ingredient formula of the sintered Nd—Fe—B permanent magnet material in mass fraction is (NdA-XREX)A(Febal-yMy)balBC, in which RE represents one or several of the rare earth elements except Nd, M represents one or several among the metal elements Al, Ga, Cu, Nb, Mo, W, V, Ta, Cr, Ti, Zr, Hf, Si, Ni, Sn, Mn, x stands for the mass fraction of RE in the whole permanent magnet material, i.e., the mass fraction of Nd replaced by RE, y stands for the mass fraction of other metals M in the whole permanent magnet material, i.e., the mass fraction of Fe replaced by other metals M, bal refers to the balance, and A %+C %+bal %=100%. The theoretical value range of A in the well-known high-performance sintered Nd—Fe—B permanent magnet material of this field varies from 26.7 to 33; however, given the loss of the RE elements in industrialized production, the value of A in practical production usually exceeds 28, and the value ranges of C, y and x are 0.5˜2, 0˜40 and 0˜10, respectively. Based on the different magnetic properties of permanent magnet to be desired, technicians of this field calculate the weight of each element actually needed according to the above formula and then gather the weighed and prepared raw materials into a group, and quickly condense them into a casting alloy through vacuum fusion. Due to the property of the rare earth metal which becomes intumescent in volume after hydrogen absorption, coarse powders may be obtained by placing the casting alloy containing rare earth metals into a hydrogen decrepitation furnace to perform the hydrogen absorption and dehydrogenation when producing the high-performance sintered Nd—Fe—B permanent magnet material.
A lot of researches and production practices have proved that, compared with other methods of hydrogen decrepitation, the performance of the magnet can be improved by dehydrogenizing the hydrogen-decrepitated coarse powders through heating. And, only when the remaining hydrogen content is below 50 ppm, it can be guaranteed that there exists no fine crack in the resultantly permanent magnet, which has even bending strength and excellent mechanical properties, from which the subsequent machining is facilitated.
Casting alloy substantially contains two compounds: main phase (RE2Fe14B) and rare earth-rich phase (Nd—Fe alloy mainly composed of Nd and other rare earth elements). Since the dehydrogenation temperature of the main phase and that of the rare earth-rich phase differs, dehydrogenation of main phase hydrides occurs at a temperature of 100° C. to 300° C., while partial dehydrogenation of rare earth-rich phase hydrides starts to occur when heated to a temperature of 350° C. to 600° C. and complete dehydrogenation occurs as the temperature is above 600° C. However, when heated to the temperature of above 600° C., a part of the main phase RE2Fe14B would generate a disproportionated reaction to produce non-magnetic or soft magnetic phases, leading to severe deteriorating in magnetic performance of the permanent magnet. Therefore, it is impossible to dehydrogenize the two phases of such two phase-integrated casting alloy separately, in order to compromise for both phases, at present, it is common to dehydrogenize at a temperature of 550° C. to 590° C. Remaining hydrogen content in the magnetic powders is approximately between 500 and 3500 ppm, after thermal insulation for 4-15 hours, and most of the rest hydrogen would be dehydrogenized in the subsequent vacuum sintering. The hydrogen content of the sintered magnet could be below 10 ppm, but due to the diffusion of hydrogen towards the outside in the process of sintering, a portion of the outside of the magnet may be hydrogenated again, or hydrogen may exist in a free form in the cracks of magnet, leading to the generation of fine cracks, which results in increased brittleness and decreased bending strength of the magnet, as well as the severely deteriorated machineable property. For the high-performance Nd—Fe—B permanent magnet material, a bulk magnet is usually cut into small pieces for use through machining, even though it is used as a whole, potential quality risk exists because of the fine cracks in the magnet. Currently, in order to prevent the defects of the magnet in mechanical properties caused by great loss of hydrogen during sintering, the hydrogen content is required to be below 50 ppm as possible at the stage of dehydrogenation of the magnetic powders, which generally can only be realized by thermal insulation for about 40 hours at a temperature between 550° C. and 590° C., leading to sufficient increase in production cost and severe decrease in production efficiency.
Therefore, the drawbacks of the existing process are as follows: when performing dehydrogenation with the existing process, there would be either incomplete dehydrogenation (i.e., hydrogen content above 50 ppm) and fine cracks in the magnet caused by the subsequent sintering, leading to increase in brittleness of the magnet, or too much time for thermal insulation may result in low efficiency and increased cost.